MUSEUM OF COMPARATIVE ZOOLOGY. 51 



ones as they do in Anodonta ; they appear larger on account of the 

 prolongation of the membranes uniting them to the mantle and to the 

 abdomen. 



It is generally understood that in Acephala the gills are formed by 

 two closely united membranes connected on the convex edge ; at the 

 upper edge of the gills these membranes separate and are attached to 

 the mantle or to the mantle and the lateral parts of the abdomen ; this 

 structure gives rise to a triangular canal, which follows the superior edge 

 of the gills and is the true gill-canal. This gill-canal is more developed 

 in the exterior pair than in the interior pair; in the interior gills it 

 seems that the internal membrane of the gills, which should form the 

 canal, is only attached to the interior and posterior portion of the 

 abdomen, and covers the intermediate portion, without, however, being 

 connected with it ; the result is that the interior gills possess a longi- 

 tudinal aperture situated at the uppermost portion of the abdomen. 

 The canal of both pairs of gills is united at the posterior extremity of 

 the gills, and forms a short common canal. The gill-canals of several 

 Acephala are constructed in this manner; more especially is this the case 

 in Anodonta. 



The result of our investigations on the structure of these organs in 

 Acepliala in general holds good also with the genus Cyclas (Sphcerium), 

 especially so as concerns the exterior gills, but an examination of the 

 interior gills shows a modification. 



That portion of the external and internal membrane of the gills which 

 forms the gill-canal, and which unites the gills to the mantle and body 

 of the animal, instead of being very narrow, as is usually the case in 

 Acephala, is so large that it overlaps the gill to the extent of double its 

 size ; consequently, in place of a narrow gill-canal, we find a cavera. 

 This cavern is anteriorly broad, posteriorly narrow, and terminates in 

 a small narrow passage, which is connected with the common gill-canal, 

 and is attached to the inferior aperture of the posterior transverse mem- 

 brane of themantle (PI. III. Fig. vii., viii., ix.); at this point there is found 

 a large crescent-shaped aperture, corresponding to the lateral aperture 

 of the interior gill-canal, as is the case in Anodonta, and this is due to 

 the fact that the internal membrane of the canal is not attached to the 

 central portion of the abdomen (PI. III. Fig. viii. g). 



The sac formed by these membranes extends on each side far beyond 

 the exterior gills, and the interior gills are attached to it ; consequently 

 it does not run parallel to the exterior gills, but runs from in front and 

 from below, to the rear and upwards, at which point it comes in contact 



